Particulate metallurgical hot topping compositions are used in the casting of metal ingots to provide an insulating cover over the top of an as-cast ingot in order to increase the feeding life of the ingot hot top. Known compositions of this type, for example, are those manufactured and sold in particulate form by Exomet, Incorporated, under the trademarks CARBON-FREE HOT TOP and RED H.T. These compositions are mildly exothermic and react in the presence of molten metal to form a light insulating layer over the ingot. The insulating layer keeps the top of the ingot molten and thereby increases soundness of the finished ingot. These compositions react with the evolution of large quantities of dense smoke obliterating safe walkways and a large portion of the pouring area from the view of cranemen.
In normal practice after an ingot has been poured, the particular material is shoveled or placed by the bag on the molten metal surface. The particulate composition flows over the metal surface thereby completely covering the surface before reacting to form the crust.
Particulate compositions of this type are formulated to react exothermically to produce an insulating covering on the surface of molten metal at the top of an ingot. Before reacting to form the covering, the compositions are in the form of loose particulate material that has been blended or formulated at room temperature and has not been subjected to elevated temperature treatment, e.g. above ambient temperature.
Conventional monolithic (pressed and sintered) solid exothermic hot tops are usually made from a particulate material such as disclosed in U.S. Pat. No. 3,144,690, sintered at low temperature (e.g. 200.degree. C or 482.degree. F) to form solid panels such as shown in U.S. Pat. No. 3,183,562 and then when contacted by the molten metal react exothermically to liberate large quantities of heat to keep the top of the ingot molten. The surface temperature of the molten metal is about 300.degree. F (1649.degree. C) at which temperature a particulate composition with conventional binders as exemplified by the above U.S. patents would react exothermically to form an irregular sintered mass that would float on the molten metal rather than form a crust. In order to form a uniform article, such compositions must be sintered at low temperature for a prolonged time. The basic compositions used for molded hot tops and other molded and cured articles are not referred to as hot topping compositions (compounds) by those skilled in the art since the art recognizes that the compositions to be moldable must have a binder.
The hot topping compositions, such as mentioned above, have been very satisfactory when used with ingots poured from molten metal produced in conventional open hearth, electric furnace, or induction furnaces. With these types of melting facilities, the ingots are allowed to stand for a long period of time before being moved to the place where the mold is stripped away from the ingot. The long delay time was necessary to allow a large portion of the ingot to solidify before it was disturbed. Moving as-cast ingots covered with conventional compositions would result in molten metal breaking open the hot topping crust and thereby exposing the metal to air reducing the effectiveness of the hot topping by increasing the solidification rate of the metal. In addition, exposing the crust to air caused it to burn and evolve more smoke. With the advent of the basic oxygen furnace, many steel mills have found it necessary to move the ingots within a short time after pouring so that the pouring platform is available for a next heat, which usually takes place within an hour of the previous pour. This "fast-move" practice has made it necessary either to delay application of the hot topping composition or to use a double application (before and after move) or use a highly exothermic composition in conjunction with the hot topping composition. These measures result in reduced efficiency and increased smoke evolution.
In some instances, it is desirable to remelt the solidified metal in the ingot hot top to further increase the yield of sound ingot metal by feeding the ingot shrink cavity. In order to do this, it is necessary to add a highly exothermic material on top of the hot topping composition to react and liberate large amounts of heat. If the hot topping composition contains volatile components, it will react and further increase the volume of smoke evolution.
In some steel mills where there is more metal produced on a given heat than there are available ingot molds, it is common practice to fill all of the ingots and then starting with the first ingot poured refill or brack-pour all of the ingots. In order to accomplish this, it is necessary to have a readily displaceable hot topping composition that evolves a minimum amount of smoke.